Currently, commercialized secondary batteries include nickel-cadmium batteries, nickel-hydrogen batteries, nickel-zinc batteries, lithium secondary batteries, etc. Among these batteries, lithium secondary batteries are in the limelight because the lithium secondary batteries have almost no memory effect compared with nickel-based secondary batteries and thus charging/discharging is freely performed, a self-discharge rate is very low, and energy density is high.
The lithium secondary battery primarily uses a lithium-based oxide and a carbon material as an anode active material and a cathode active material, respectively. The lithium secondary battery includes an electrode assembly in which an anode plate and a cathode plate coated with the anode active material and the cathode material, respectively, are arranged with a separator therebetween, and an exterior material sealing the electrode assembly and an electrolyte together.
Generally, lithium secondary batteries may be classified into can-type secondary batteries in which an electrode assembly is mounted within a metallic can, and pouch-type secondary batteries in which an electrode assembly is mounted within a pouch including an aluminum laminated sheet depending on the shape of an exterior material.
Recently, secondary batteries are widely used for medium and large apparatuses such as automobiles or power storing apparatuses as well as small apparatuses such as portable electronic apparatuses. In the case where the secondary batteries are used for medium and large apparatuses, lots of secondary batteries are electrically connected in order to increase capacity and power. Particularly, pouch-type secondary batteries are widely used for the medium and large apparatuses due to an advantage of easy stacking. Also, a plurality of secondary batteries may be stored in a pack case to configure one battery pack.
The battery pack requires various characteristics. Safety is a representative characteristic among the characteristics. Furthermore, since it is considered that safety of the battery pack mounted on an automobile is directly linked to life of a passenger, the safety of the battery pack is very important.
Particularly, one of important issues relating to the safety of the battery pack is direct exposure to a flame. In the case of the battery pack, although preventing fire from occurring in the inside of the battery pack is a first priority, fire may occur in the inside of the battery pack depending on cases. In this case, when a flame or a high temperature gas is exposed much to the outside of the battery pack, the exposed flame or high temperature gas may destroy or damage other elements in the vicinity of the battery pack and cause secondary fire. Particularly, when fire occurs in the inside of the mounted battery pack, the occurred fire may be very dangerous for a passenger. Furthermore, in the case where the battery pack is mounted on a hybrid automobile or collides with another automobile, when fire or a high temperature gas is exposed to a fuel storing tank, explosion of fuel may be caused and thus a big accident may be generated.
Also, another of important issues relating to the safety of the battery pack is impact resistance. That is, it is preferable that internal elements of the battery pack are not easily destroyed even by an impact of a preset level or more and maintain their performance. Particularly, since a battery pack for an automobile is always exposed to a danger of automobile collision, a characteristic that internal elements of the battery pack are not easily destroyed from an accident is required even more.